This invention relates to a low temperature isomerization process.
Isomerization of normal paraffins is one of several widely used refining processes for upgrading low value, low octane hydrocarbons to higher value, higher octane hydrocarbons for use in motor fuels and other applications. Isomerization of light normal paraffins, especially those with 4-7 carbon atoms, has become especially attractive in recent years with the development of chloride- or bromide-activated alumina or platinum alumina catalysts which have much greater activity than the older catalysts. This improvement has permitted operation at low temperatures, for instance less than 400.degree. F. (204.degree. C.) where the thermodynamic equilibrium is more favorable for the isoparaffins and has made possible both high conversion and high selectivity for the desired reactions.
As is the case with most catalytic reactions, over a period of time the activity of the catalyst declines. In this instance the decline is due to the poisoning effects of trace feed contaminants such as sulfur compounds and water and to the production of a small amount of coke which tends to obscure the catalyst surface. It is normal practice to operate two catalyst beds in series so that the first bed picks up most of the catalyst poisons and suffers the greatest decline in activity. It is also normal practice to offset this gradual activity decline by gradually raising reactor temperature. This procedure raises catalyst activity and feed conversion while decreasing selectivity and increasing the production of coke.
The isomerization process is generally carried out in the presence of hydrogen, with hydrogen/hydrocarbon feed molal ratios of 0.25:1 to 10:1 generally being preferred. The use of hydrogen has generally been found helpful in sustaining catalyst activity and in retarding cracking reactions and the production of coke.